1. Field of the Invention
The present invention relates to a high-average-power fiber laser system which utilizes a multi-mode dual-clad fiber amplifier in order to generate a higher average power which includes compensation for undesirable multiple modes by way of phase conjugation.
2. Description of the Prior Art
High power laser weapon systems are generally known in the art. An example of such a high power laser system is disclosed in commonly owned U.S. Pat. No. 5,198,607. Such laser weapon systems normally include a high power laser weapon and a tracking system for locking the high power laser on a target, such as a ballistic missile, cruise missile, bomber or the like. Such laser weapons are used to destroy or "kill" such targets.
Such laser weapon systems are known to employ relatively large chemical lasers. However, such chemical lasers have several drawbacks. For example, such chemical lasers are relatively bulky and relatively inefficient and require highly reactive chemical fuels. As such, a need has developed to provide relatively efficient compact laser weapons. Although such known fiber laser systems are relatively compact and efficient, such fiber lasers have heretofore had insufficient power output for use in laser weapon systems. More particularly, such fiber lasers are known to include a single mode dual-clad optical fiber. More particularly, the optical fiber includes a core, for example, formed from SiO.sub.2 and doped with a rare earth metal, such as, Yb or Nd, Er or other rare earth ions. The doped core is clad by two different cladding layers having different indices of refraction to cause the total internal reflection of the light within the optic fiber to form a single mode fiber. Examples of such optical fibers used for fiber laser are disclosed in U.S. Pat. Nos. 4,815,079; 5,087,108; 5,218,665; 5,291,501; 5,461,692; 5,530,709; and 5,566,196. Such fiber lasers are known to be diode pumped and generate relatively low average power levels, for example, up to 50 watts, which is limited by optically induced damage in the small single-mode core. Unfortunately, such relatively low power levels of fibers lasers have made them unsuitable for some applications including defense applications.
Various attempts have been made to increase the average power output of such fiber lasers. Examples of such attempts are disclosed in U.S. Pat. Nos. 5,121,460 and 5,373,576. Such attempts generally involve the use of relatively complex optical fibers. For example, the '460 patent teaches the use of an optical fiber having a neodymium doped primary core surrounded by a first elliptically shaped multi-mode cladding of fused silica. A samarium doped secondary core is formed around the primary core within the first layer of cladding. The secondary core is utilized for suppressing higher order modes.
The '576 patent also discloses the use of a relatively complex high average power optical fiber. More particularly, the '576 patent discloses an optical fiber formed, with a doped core surrounded by a first multi-mode cladding layer formed from, for example, fused silica. A second cladding layer is formed around the first cladding layer and formed from a cross-link polymeric material having a liquid component. The optical fibers disclosed in the '460 and '576 patents are relatively complex. Thus, there is a need for a relatively high average power fiber laser that utilizes relatively less complex optical fibers than known systems.